Hardware and such… Stanley  You cannot say you received no warning of endless classification!  “Simple” is a term covered on the last page, but “Double Acting” requires explanation.  When the steam pushes the piston in both directions, that’s double acting (DA).  The drawing illustrates a single cylinder, DA engine.  Steam (in red) enters the “steam chest” (a receptacle to store steam about to enter the engine) where a valve alternately directs the steam to the top and bottom of the piston, pushing it in both directions.  The piston thrust is transferred via the piston rod to the crosshead and then onto the crankshaft.  “Packing or stuffing”           (see below) prevents steam from escaping around the piston rod and the crosshead prevents the sideways torque of the connecting rod from bending the piston rod. From the left hand image it is easy to see how the shape of the slide valve led to the term “D” valve.  Steam (red arrow) enters the cylinder upper end when the valve approaches the extreme downward position and exposes the upper port. Meanwhile, the valve inner edge uncovers the lower cylinder port and the exhaust steam is directed to the central exhaust passage by the valve concave inner surface.   The ‘D” valve is really four valves in one, the outside edges control steam flow to the cylinder while the inside edges control exhaust from the cylinder. (Simple, Double Acting, Slide Valve Steam Engine)